Active reconfigurable stretch forming

ABSTRACT

This invention concerns an active reconfigurable stretch forming tool, and in another aspect the invention is a method of stretch forming. The tool comprises an array of extensible shape forming elements which are driven in extension to produce the same force per unit area across a workpiece during shape forming. An array of limit switches are located in front of the array of shape forming elements, such that each shape forming element is driven in extension towards a respective limit switch during shape forming. In use, each limit switch is activated by the workpiece as it is shaped and each switch, upon activation, prevents further extension of the respective driven element. The tool and method are useful in the forming of three dimensional shapes in solid sheet metal or mesh, to produce panels for reflector antennas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application a continuation of U.S. application Ser. No. 12/162,317,which is a filing under 35 U.S.C. §371 of International PatentApplication PCT/AU2007/000059, filed Jan. 23, 2007, which claimspriority to Australian application no. AU 2006900369, filed Jan. 25,2006, each of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

In a first aspect the invention is an active reconfigurable stretchforming tool, and in another aspect the invention is a method of stretchforming. The tool and method are useful in the forming of threedimensional shapes in solid sheet metal or mesh, to produce panels forreflector antennas.

BACKGROUND ART

The manufacture of accurate antenna panels remains one of the mostdifficult and labour intensive aspects of large-scale reflector antennamanufacture, and one that has a significant impact on antennaperformance.

A number of methods have been employed in the manufacture of antennapanels. Some of these methods aim for high-accuracy construction at theexpense of speed and cost, while others are more suited to massmanufacture of less accurate parts. Some of the more widely used methodsare outlined below:

Bed of Bolts

The “bed of bolts” methods involves laying sheetmetal strips over anarray of adjustable bolts attached to a large flat table. The bolts areadjusted in height to represent the curvature required.

Strips of sheetmetal sufficiently narrow to take the required curvaturewith only elastic deformation are laid across the tops of the bolts andare then pulled down by vacuum bagging. Whilst the strips are held inshape by a modest vacuum, a rigid backing structure is bonded to theopen side, to hold the strips permanently in the formed shape.

While this method produces highly accurate panels of any desired shape,the presence of elastic deformation stresses in the material requires aclosely spaced array of backing members to hold the panel in shape. Thevacuum used to hold the panel can lead to shallow regularly spaceddimples between the bolts, and the regular spacing of the backingmembers has been known to cause periodic ripples. Both of these issueshave caused problems with antenna grating lobes. The Bed of Bolts methodis described more fully in CSIRO's patent for rapidly setting theheights of the adjustable bolts [1].

Press Forming

Press forming involves compressing a sheet of material between shapeddies. The material is deformed plastically so that it permanentlyretains the pressed shape. Depending on the shape being produced, thematerial may be either plastically stretched or compressed, or both,during forming. Some spring back or “recovery” occurs after the pressingforces are removed, so the shape of the forming dies is not necessarilythe same as the shape of the completed panel.

Forming an accurate shape free from wrinkles and buckles is complex andmay involve a large number of iterations to the shape and details of theforming dies. The dies are typically made from hardened tool steel, arelarge and expensive, and may only produce one shape each. Large pressesup to many hundreds of tonnes in capacity are required to operate thedies. However, once the dies have been proven, production of repetitionparts is extremely fast.

Hydroforming

Hydroforming involves stretching a flat panel into a shaped die underhydraulic pressure. The material then retains the shape of the die. Likepress forming, the material will recover to some degree after forming.The hydroforming process for manufacture of antenna dishes has been putto commercial use by Anderson Manufacturing Inc. in the United States.

Hydroforming dies are large, but simple in comparison to press formdies, and may be made from soft materials or backed with polymer fillingcompounds to simplify shaping. No large press is required. Extremelylarge panels may be produced, but the die, once corrected and proven,will only produce parts of one shape, and variations in the propertiesof the workpiece material may affect the repeatability of the recoveryafter forming.

Recent efforts in the United States aimed at developing antennasolutions for radioastronomy have resulted in the successfulhydroforming of numerous 6 metre dishes for the Allan Telescope Array,and investigations into hydroforming reflectors with a diameter of 12metres are continuing.

Stretch Forming

The term Stretch Forming covers a number of areas of metal forming, fromthe shaping of curved beams to the shaping of panels for aircraft andautomotive bodies. Like press forming and hydroforming, a shaped die orstretch form tool is required.

In the case of stretch forming of sheet material, the sheet is stronglygripped along two opposing edges and supported above a shaped formblock. The form block is then driven up underneath the tightly stretchedsheet (or the grippers move downwards), until the shape of the form toolis reproduced in the material, in a manner similar to stretching a sheetof thin rubber over, say, a football. This is illustrated in FIGS. 1(a)-(c). In FIG. 1( a) a sheet of material is shown gripped above a formblock for stretching. In FIG. 1( b) stretching load is applied by thegrippers and the form block is moved relative to the sheet to the pointof contact. In FIG. 1( c) forming is complete.

The simultaneous application of stretching and forming forcessignificantly reduces, and can almost eliminate, the shape recovery ofthe material after forming. The mechanism through which this is achievedis illustrated in FIG. 2.

In FIG. 2( a), a piece of material has been deformed by application of abending load. Tensile and compressive stresses are generated within thematerial as it is bent. These stresses increase in magnitude towards theoutside faces of the material and there is a neutral axis in the centrewhere no tensile or compressive stresses exist.

All materials will recover elastically to some degree after plasticdeformation, in a direction opposing that of the applied deformationstresses. In this case the uneven distribution of stresses will causethe material to straighten slightly after the bending load is removed,and the final curvature will be noticeably less than intended.

In FIG. 2 b, the material has been both bent and stretched along its ownaxis. If the stretching load is sufficient to cause yielding or slightplastic deformation in this direction, the stresses within the materialwill change to an even distribution of tensile stress. Later, when thestretching load is removed, the elastic recovery occurs along thecentreline of the material, with little or no change in overall shape.

An hydraulically powered machine, called a stretch former, is used tocarry out this process. It consists of a base or table on which thestretch form tool is mounted, and an array of grippers on two sides thathold the edges of the workpiece while it is being stretched over theform block. The grippers simultaneously apply a sufficiently largestretching load to cause the workpiece material to yield across its fullwidth. Stretch formers are relatively common in industrial use.

Stretch forming has traditionally been performed over solid form blocks,made from metal, hard plastics and occasionally wood where shapes aremodest and accuracy is not critical.

Stretch forming is a fairly fast process, but the need for manufacturedform tools and the limitations imposed by form blocks with fixed shapes,have prompted development of reconfigurable tools consisting of an arrayof adjustable elements that can be set to form an approximation to acontinuous curved surface, in a manner similar to the Bed of Boltsdescribed above.

A representation of a reconfigurable stretch form tool with a 6×6 arrayof adjustable elements is shown in FIG. 3. In practice the elements aretypically domed on their working faces, rather than flat-ended as shown.

As the surface of the reconfigurable form block is composed ofindividual facets rather than a continuous surface, a layer ofconformable material such as a sheet of polymer rubber is laid over thetop of the form tool to prevent dimpling of the workpiece. This layer isknown as an interpolator.

A number of papers have been published detailing the development andapplication of this technique to the manufacture of repair parts foraircraft, from both sheet metal and composite materials. [2], [3], [4].

Of these papers, [2] and [3] discuss aspects of the actuation andcontrol of the elements of a reconfigurable stretch form tool, using2688 individual moveable elements with servo motor and lead screwcontrol, to set the positions of the adjustable elements before lockingthem into place and using the tool as a conventional fixed form tool. Anumber of patents exist covering aspects of construction and control ofthis type of system. [5], [6], [7], [8].

In [4], Walczyk notes that where composite materials are concerned,automatic lay-up machines can be used to prepare composite parts overthe top of reconfigurable tools in the flat state, with subsequentactive driving of the reconfigurable tool from below the part to formthe required curvature.

A number of drawbacks are apparent in these methods:

Where large tools with many elements are concerned, the task ofcontrolling thousands of individual elements is difficult. Each elementis subject to a proportion of the total force applied by the stretchforming machine, so the elements must be robust or they are likely to beunreliable in positioning or repeatability.

Where panels of non-uniform curvature are involved, the effectivepressure between the interpolator and the workpiece material can vary,resulting in differing degrees of compression of the interpolator overthe top of the form tool elements. This results in a departure of theshape of the formed part from the nominal surface defined by the toolelements.

And lastly, as the entire forming load is supported by the structure ofthe form tool and its elements, any deformation of the tool structureunder load will be replicated in the shape of the workpiece.

DISCLOSURE OF THE INVENTION

A first aspect of the invention is an active reconfigurable stretchforming tool for forming a three dimensional shape in a solid sheetmetal or mesh workpiece, to produce a panel for a reflector antenna. Thetool comprises:

An array of extensible shape forming elements which are driven inextension to produce the same force per unit area across a workpieceduring shape forming.

And an array of limit switches located in front of the array of shapeforming elements, such that each shape forming element is driven inextension towards a respective limit switch during shape forming.

Wherein, in use, each limit switch is activated by the workpiece as itis shaped and each switch, upon activation, prevents further extensionof the respective driven element.

The array of limit switches defines the shape to be imparted to theworkpiece. The active reconfigurable tool achieves shape control of theworkpiece by directly measuring the workpiece during shaping. The toolalso permits variation of the shape produced, and facilitates correctionof systematic shape-forming errors, such as deformation of the toolstructure or compression of an interpolator. Further, the tool mayincorporate shape-control feedback or error correction as shapingproceeds.

The tool may be used in a conventional industrial stretch formingmachine, with no significant modifications to the machine's usual set-upor operation. For instance, the conventional opposing sets of workpiecegrippers may be used.

The shape forming elements may comprise hydraulic cylinders and ramseach of which is powered from a single hydraulic power supply. Since thehydraulically powered elements are connected via hydraulic lines to asingle power supply, the hydraulic pressure in the cylinders will beequalised. This prevents any one cylinder causing localised excessivedeformation of the workpiece.

Each ram may be surmounted by a tilting pad and each tilting pad may beinterlocked with its adjacent pads to form a continuous articulatedsurface. As a result of using the tilted pads the array of elements maybe sparse compared to a conventional reconfigurable stretch formingtool. The tilting pads may be provided with a spherical seat to fitspherical ends on the hydraulic cylinder rams.

An interpolator may be located on the articulated surface to receive theworkpiece.

The rams will generally be arranged below the workpiece to produceconcave workpieces. An extension of the invention is to place an arrayof rams both above and below the workpiece. This will allow theproduction of panels with both concave and convex curvature.

The limit switches may be aligned vertically over respective tiltingpads. Other locations for the limit switches may be used, provided theycan be actuated by the movement of the workpiece, interpolator, or ram,as the formation of the workpiece shape proceeds. Each switch may beconnected to a simple solenoid valve in the hydraulic line leading toits respective cylinder. As the workpiece is shaped it will contact oneor more of the limit switches, and as soon as this occurs the switchoperates to close the solenoid valve and prevent further movement of therespective tilting pad.

The switches themselves may be simple On-Off mechanical switches.Alternatively, the switches may be constant-contact analogue devices,and they may be programmed or set to trigger at the appropriate height.As a result it may be possible to implement multi-staged forming, wherea panel is formed to initial,-intermediate-final, or roughing-finishingstages. This graduated approach may be beneficial where deep shapes orhigh accuracy, or both, are required, by avoiding excessive stretchingor the possibility of buckling in any one stage.

To further approximate a continuously curved surface the shapingsurfaces of the tilting pads may be formed with a spherical radiusapproximating the curvature of the required panel.

A number of sets of tilting pads with a range of spherical radii may beprovided for the tool. Alternatively, the top of each tilting pad couldbe made flat, with provision for clipping inserts of varying sphericalradius into place.

Where an individual panel has areas of high and low curvature, a set ofpads with appropriate incrementally different radii could be clipped tothe stretch form tool to accommodate such variations.

Another aspect of the invention is a method for forming threedimensional shapes in a solid sheet metal or mesh workpiece, to producea panel for a large reflector antenna. The method comprises:

-   -   Stretching a metal workpiece in a flat state in front of an        array of extensible shape forming elements.    -   Driving each shape forming element of the array in extension to        produce the same force per unit area across the workpiece, to        form a shape in the workpiece. Extending each shape forming        element towards a respective limit switch during shape forming        until the workpiece activates the limit switch.    -   Preventing further extension of a shape forming element upon        activation of the respective limit switch.

Unlike other implementations of reconfigurable stretch forming tools,simultaneous position control of very large numbers of driven elementsis not required. In this invention the shape forming elements are drivento produce the same force per unit area across the workpiece, and thedistribution of power and the element positions during stretching iscontrolled by the natural behaviour of the workpiece material.

The shape forming elements may comprise hydraulic rams, and the methodmay produce panels of any curvature within the travel available in thehydraulic rams.

The final position of the shape forming elements, and therefore thepanel shape, though accurately controlled by the array of limitswitches, is “dumb” and requires no active intervention by a controlsystem. It is anticipated that the setting of the limit switch arraywill be performed according to the method described in [1].

Variations between measured and theoretical panel shapes may beaccommodated in the settings of the limit switch array. If the limitswitch array is also used for shape measurement, it may be possible toimplement an automatic process with closed-loop shape control.

Using the invention, large sections of panel may be formed from onepiece of material, eliminating the time and labour involved in laying upthe numerous individual strips required by the bed of bolts method.

The use of one-piece panels rigidly formed to an accurate shapeeliminates the need for multiple pre-formed backing ribs to hold thepanel's shape, and the need for the ribs to be aligned with the jointsbetween individual strips. This will allow the backing structure to bedesigned for stiffness and economy without constraints imposed by thelayout or curvature of the panel.

The method makes use of existing metal forming machinery and techniques,off the shelf parts, and a simple control system.

The method proposed offers significant improvements in terms of cost andversatility while maintaining equivalent surface accuracy to the bestmethods currently available.

BRIEF DESCRIPTION OF THE DRAWINGS

The prior art has been described above with reference the followingdrawings in which:

FIGS. 1( a)-(c) are a series of schematic diagram illustrating stretchforming. In FIG. 1( a) a sheet of material is shown gripped above a formblock for stretching. In FIG. 1( b) stretching load is applied by thegrippers and the form block is moved relative to the sheet, to the pointof contact. In FIG. 1( c) forming is complete.

FIG. 2( a) is a diagram showing the distribution of tensile andcompressive stresses during bending of a piece of material.

FIG. 2( b) is a diagram showing the distribution of tensile-onlystresses during stretch forming.

FIG. 3 is a diagram showing a reconfigurable stretch-form tool with a 6x 6 array of adjustable elements.

An example of the invention will now be described with reference to thefollowing drawings, in which:

FIGS. 4( a)-(c) are a series of diagrams illustrating the principle ofoperation of an active stretch forming tool. FIG. 4( a) shows areconfigurable stretch tool in an industrial stretch forming machinebefore stretching commences FIG. 4( b) shows the reconfigurable stretchtool at an intermediate point of stretching. FIG. 4( c) shows the toolwhen stretching is completed.

FIG. 5( a) is a diagram showing a tilting pad for the tool of FIGS. 4(a)-(c). FIG. 5( b) is the pad of FIG. 5( a) inverted.

FIG. 6 is a diagram showing three interlocking pads forming anarticulated surface.

FIG. 7( a) illustrates a model of an array of rams and tilting padsbelow an array of stops. FIG. 7( b) shows how the array of pads tilt andorient to form the curve defined by the stops when they are brought intocontact with each other.

BEST MODES OF THE INVENTION

Referring now to FIG. 4( a) reconfigurable stretch forming tool 10involves a sparsely populated array of elements 12. Each element 12comprises an hydraulic cylinder 14 all of which are powered from asingle hydraulic power supply 16. An hydraulic ram 18 may be drivenupwards by each cylinder 14. The tool may be used in a conventionalindustrial stretch forming machine, with no significant modifications tothe machine's usual set-up or operation.

The span between elements 12 is much larger than that in thereconfigurable tools previously described, and each ram 18 is surmountedby a tilting pad 20. Each tilting pad 20 is interlocked with itsadjacent pads to form a continuous articulated surface indicatedgenerally at 22. A polymer interpolator 24 is placed between the pads 20and the workpiece 26 which is held by grippers 28 and 30.

Rather than providing a fixed, pre-set surface over which a sheet orworkpiece material is stretched, as in a stretch forming machine, thematerial 26 is held stretched in the flat state while the rams 18 of thereconfigurable tool are driven upwards, so forming a three dimensionalshape in the panel.

The hydraulically powered elements 12 are not individually controlled.As they arc connected via hydraulic lines to a single power supply 16,the hydraulic pressure in the cylinders will be equalised. This preventsany one cylinder causing localised excessive deformation of theworkpiece 26.

Above the workpiece 26 is suspended an array of limit switches 32,aligned vertically over each active element 12. Each switch 32 isconnected to a simple solenoid valve 34 in the hydraulic line leading toits relevant cylinder 14. The switches 32 themselves may be simpleOn-Off mechanical limit switches of the types often used in industrialmachinery, where switching occurs on contact. Alternatively, theswitches may be constant-contact analogue devices like linear voltagedifferential transducers (LVDTs), programmed or set to trigger at theappropriate height.

If such a programmable device is used in place of an On-Off switch, itmay be possible to implement multi-staged forming, where a panel isformed to initial,—intermediate-final, or roughing-finishing stages.This graduated approach may be beneficial where deep shapes or highaccuracy, or both, are required, by avoiding excessive stretching or thepossibility of buckling in any one stage.

As the workpiece 26 rises, areas of the workpiece will make contact withsome of the limit switches 32, as shown in FIG. 4( b), closing off thesolenoid valve for the cylinder at that point and preventing furthermovement. When all the solenoid valves have been closed in this way, asshown in FIG. 4( c), the forming process is complete.

Final stretching of the workpiece after all active elements havecontacted their respective limit switches, will equalise internalstresses within the workpiece material, and ensure that its formed shapeis retained after relaxation of all forming forces, and removal of theworkpiece from the stretch forming machine.

The positions of the array of limit switches defines the shape of theworkpiece that will be produced. It is anticipated that the setting ofthe limit switch array will be performed according to the methoddescribed in [1].

Tooling Details.

The array of tilting pads 20 used on the ends of the hydraulic cylinderrams 18, interlock with each other. In this way a continuous articulatedforming surface 22 is created. The interpolator sits on the relativelycontinuous surface 22, and the combined effect is to prevent localisedhigh-spots that could dimple the workpiece 26 between points measured bythe limit switch array.

The tilting pads 20 are provided with a spherical seat 36 on one side tofit spherical ends 38 on the hydraulic cylinder rams 18. A simple wirecirclip can be used to retain the pads on the rams after forming, whenthe hydraulic cylinders retract to their rest position.

To further approximate a continuously curved surface and to assist theinterpolator 24 to produce smooth workpiece curvature, the uppersurfaces of the tilting pads 20 are formed with a spherical radiusapproximating the curvature of the required panel.

If the range of panels to be stretch formed requires widely varyingradii of curvature, a number of sets of tilting pads with a range ofspherical radii, differing by, say, 1 m increments, could be fitted tothe tool as required.

Alternately, the top of each tilting pad could be made flat, withprovision for clipping inserts of varying spherical radius into place.

Where an individual panel has areas of high and low curvature, a set ofpads with appropriate incrementally different radii could be clipped tothe stretch form tool to accommodate such variations.

FIG. 5 shows a possible design of tilting pad 20, and illustrates thefeatures 38 and 40 that interlock with adjacent pads to form anarticulated surface, and the socket for mounting the pad on thehydraulic ram. FIG. 6 illustrates the interlocking of a number of pads20.

FIG. 7( a) illustrates a model of an array of rams and tilting pads 42below an array of stops 44. FIG. 7( b) shows how the array of pads 42tilt and orient to form the curve defined by the stops 44 when they arebrought into contact with each other.

Stresses Developed in Reconfigurable Tools.

In one example, an antenna of 15 m diameter with an f/d of 0.4 gives afocal length of 6 m. As a the minimum instantaneous radius of a parabolaequals twice the focal length, it is necessary to stretch form of asection of a spherical surface with a radius of 12 m, from aluminiumsheet with a thickness of 1.2 mm. The material considered is grade5005-H34, which has a yield stress of 138 MPa [9].

This stretch forming process is analogous to hydroforming, wherehydraulic pressure is used to deform a flat sheet. If allowed to proceedunrestrained, both processes will tend to produce a spherical radius. Asthe tensile stresses in the wall of a spherical vessel subject tointernal hydraulic pressure are equal in all directions, and the tensilestresses are proportional to the pressure, treatment of stretch formingas a hydraulic pressure problem is sufficiently valid to check theviability of the proposed stretch forming process.

In this case, the yield stresses generated in the workpiece by thestretch forming grippers are equivalent to the tensile stresses in thewalls of a pressure vessel. Therefore the contact pressure on any of thetilting pads is equivalent to the internal pressure in a vessel of thesame radius with the same tensile wall stress.

Tensile stress in a thin-walled spherical pressure vessel equals:

f=Pr/2t

Where:

f=stress (MPa);

P=internal pressure (MPa);

r=radius of vessel (m) and

t=wall thickness (m).

For a vessel of radius 12 m, with a wall thickness of 1.2 mm and atensile wall stress of 138 MPa, the equivalent internal pressure istherefore 0.276 MPa. This is the nominal surface pressure that would bepresent on a tilting pad to stretch form a panel to a radius of 12 m.

A model of a possible tilting pad design subject to this load wasstudied using a linear mechanical finite element analysis package,COSMOSXpress, and the results show the maximum stress developed in thispart is approximately 8.5 MPa. If the pad is made from mild steel with ayield stress of 250 MPa, this represents a factor of safety in thedesign of at least 29.

The load carried by the tilting pad will also be supported by thehydraulic cylinder. If a cylinder with a piston diameter of 75 mm isassumed, the hydraulic pressure required can be found.

The pressure load of 0.276 MPa on the top of the tilting pad, togetherwith the top face area, 0.019 m2, indicates a normal load on one ram of5.25 kN. The hydraulic pressure necessary to produce this load on a 75mm piston is 1.19 MPa. When allowances for losses are considered, aminimum system pressure of approximately 2.5 MPa is required. Industrialhydraulic systems built from off-the-shelf parts typically operate atsystem pressures ranging from 20 MPa to 60 MPa, so the hydraulicpressure requirements are very modest.

Another area considered was the bending stress applied laterally to therams of the hydraulic cylinders by frictional resistance as theworkpiece and or interpolator slides across the tops of the pads duringstretching. It is anticipated that the interpolator would be a type ofurethane rubber. These materials are available in a wide range ofcompounds with varying degrees of hardness. Manufacturers ofpolyurethanes for coatings on conveyer rollers claim the coefficient offriction (μ) for these materials can be tailored to suit theapplication, with μ=0.4 being a minimum value.

To cover all eventualities, a worst case coefficient of friction ofμ=1.0 was assumed, as was a cylinder ram with a diameter of 50 mm,cantilevered with a free length of 250 mm.

As before, an axial load on one tilting pad of 5.25 kN is assumed. Whenμ=1.0, the lateral load at the tip of the ram will also be 5.25 kN,applied simultaneously with the axial cylinder load. Analysis of a modelrepresenting an allow steel ram from a hydraulic cylinder under thiscombined load gives a factor of safety around 6. Whilst the factor ofsafety in the ram is lower than that in the tilting pad, this briefanalysis demonstrates that the stresses generated in the majorcomponents of a tool to implement this concept are modest, and thatimplementation is feasible. Optimisation of the design and selection ofappropriate hydraulic components will lead to a reliable a robustsystem.

REFERENCES

[1] U.S. Pat. No. 5,976,287 “Method and Apparatus of Stud Array UpstandSetting”, Parsons, Barker, Yabsley, Kesteven, Bird, Harrigan. Nov. 21999.

[2] Daniel F. Walczyk, Yong-Tai lm, Rensselaer Polytechnic Institute,“Hydraulically-Actuated Reconfigurable Tool for Flexible Fabrication:Implementation and Control”. Transactions of the American Society ofMechanical Engineers, Volume 122, pp 562-568. August 2002.

[3] John M. Papazian, Northrop Grumman Corporation, “Tools of Change”,Mechanical Engineering Online Magazine, American Society of MechanicalEngineers, February 2002.

[4] Daniel F. Walczyk, Jean F. Hosford, Rensselaer PolytechnicInstitute, John M. Papazian, Northrop Grumman Corporation. “UsingReconfigurable Tooling and Surface Heating for Incremental Forming ofComposite Aircraft Parts”. Journal of Manufacturing Science andEngineering, Volume 125, pp 333-343, May 2003.

[5] U.S. Pat. No. 6,012,314 “Individual Motor Pin Module”, Sullivan etal, Jan. 11 2000

[6] U.S. Pat. No. 6,053.026 “Block Set Form Die Assembly”, Nardiello etal, Apr. 25 2000

[7] U.S. Pat. No. 6,089,061 “Modularised Reconfigurable Heated FormingTool”, Haas et al, Jul. 18 2000

[8] U.S. Pat. No. 6,578,399 “Single Die Modularised ReconfigurableHoneycomb Core Forming Tool”, Haas et al, Jun. 17 2003

[9] http://www.matweb.com/index.asp?ckck=1 Accessed September 2005.

Although the invention has been described with reference to a particularexample, it will be understood that it may be extended to place an arrayof rams is both above and below the workpiece. This will allow theproduction of panels with both concave and convex curvature.Alternatively, the invention may also be used with designs that locatepanel joints along lines of inflection between concave and convex areas.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

1. An active reconfigurable stretch forming tool for forming a threedimensional shape in a solid sheet metal or mesh workpiece, to produce apanel for a reflector antenna; the tool comprising: an array ofextensible shape forming elements which are driven in extension toproduce the same force per unit area across a workpiece during shapeforming; and, an array of limit switches located in front of the arrayof shape forming elements, such that each shape forming element isdriven in extension towards a respective limit switch during shapeforming; wherein, in use, each limit switch is activated by theworkpiece as the workpiece is shaped and each switch, upon activation,prevents further extension of the respective driven element.
 2. Theactive reconfigurable stretch forming tool according to claim 1, whereinthe array of limit switches defines a shape to be imparted to theworkpiece.
 3. The active reconfigurable stretch forming tool accordingto claim 1, wherein the tool incorporates shape-control feedback orerror correction during shape forming.
 4. The active reconfigurablestretch forming tool according to claim 1, wherein the shape formingelements comprise hydraulic cylinders and rams each, of which is poweredfrom a single hydraulic power supply.
 5. The active reconfigurablestretch forming tool according to claim 4, wherein each ram issurmounted by a tilting pad and each tilting pad is interlocked with itsadjacent pads to form a continuous articulated surface.
 6. The activereconfigurable stretch forming tool according to claim 5, wherein thetilting pads are provided with a spherical seat to fit spherical ends onthe rams of the hydraulic cylinders.
 7. The active reconfigurablestretch forming tool according to claim 5, wherein an interpolator islocated on the articulated surface to receive the workpiece.
 8. Theactive reconfigurable stretch forming tool according to claim 7, whereinthe rams are arranged below the workpiece to produce concave workpieces.9. The active reconfigurable stretch forming tool according to claim 8,wherein there is an array of rams both above and below the workpiece.10. The active reconfigurable stretch forming tool according to claim 5,wherein the limit switches are aligned vertically over respectivetilting pads.
 11. The active reconfigurable stretch forming toolaccording to claim 1, wherein each limit switch is connected to a simplesolenoid valve in a hydraulic line leading to its respective cylindersuch that, as the workpiece is shaped the workpiece will contact one ormore of the limit switches, and as soon as the workpiece contacts one ormore of the limit switches the switch operates to close the solenoidvalve and prevent further movement of the respective tilting pad. 12.The active reconfigurable stretch forming tool according to claim 11,wherein the limit switches are constant-contact analogue devices. 13.The active reconfigurable stretch forming tool according to claim 12,wherein the limit switches are programmed or set to trigger at apredetermined height.
 14. The active reconfigurable stretch forming toolaccording claim 13, wherein the shaping surfaces of the tilting pads areformed with a spherical radius approximating the curvature of a requiredpanel.
 15. The active reconfigurable stretch forming tool according toclaim 14, wherein a number of sets of tilting pads with a range ofspherical radii are provided for the tool.
 16. The active reconfigurablestretch forming tool according to claim 13, wherein the top of eachtilting pad is flat with provision for clipping inserts of varyingspherical radius into place.
 17. A method for forming three dimensionalshapes in a solid sheet metal or mesh workpiece, to produce a panel fora large reflector antenna, the method comprising: stretching a metalworkpiece in a flat state in front of an array of extensible shapeforming elements; driving each shape forming element of the array inextension to produce the same force per unit area across the workpiece,to form a shape in the workpiece; extending each shape forming elementtowards a respective limit switch during shape forming until theworkpiece activates the limit switch; and, preventing further extensionof a shape forming element upon activation of the respective limitswitch.
 18. The method according to claim 17, comprising the furtherstep of applying shape-control feedback or error correction as shapingproceeds.
 19. The method according to claim 18, wherein each limitswitch is connected to a simple solenoid valve in the hydraulic lineleading to its respective cylinder such that, as the workpiece is shapedthe workpiece will contact one or more of the limit switches, and assoon as the workpiece contacts the one or more of the limit switches theswitch operates to close the solenoid valve and prevent further movementof the respective tilting pad.
 20. The method according to claim 19,wherein, the limit switches are programmed or set to trigger at apredetermined height.